Nanomechanical switches and circuits

ABSTRACT

A highly miniaturized nanomechanical transistor switch is fabricated using a mechanical cantilever which creates a conductive path between two electrodes in its deflected state. In one embodiment, the cantilever is deflected by an electrostatic attraction arising from a voltage potential between the cantilever and a control electrode. In another embodiment, the cantilever is formed of a material with high magnetic permeability, and is deflected in response to complementary magnetic fields induced in the cantilever and in an adjacent electrode. The nanomechanical switch can be fabricated using well known semiconductor fabrication techniques, although semiconductor materials are not necessary for fabrication. The switch can rely upon physical contact between the cantilever and the adjacent electrode for current flow, or can rely upon sufficient proximity between the cantilever and the adjacent electrode to allow for tunneling current flow.

This application claims priority under 35 U.S.C. §119 (e)(1) ofprovisional application No. 60/171,910, filed Dec. 23, 1999.

FIELD OF THE INVENTION

This invention relates generally to submicron switching devices and morespecifically to a nanomechanical switch using a deformable cantileverelement.

BACKGROUND OF THE INVENTION

A significant factor in the electronic revolution has been the steady toevolution of increasingly smaller integrated circuit geometries for thefabrication of semiconductor switching transistors. Typical featuresizes have been reduced from tens of microns in the early eighties, toroughly ten microns in the mid eighties, to below one micron in the midnineties, until minimal lateral feature sizes of as small as 0.15microns are not uncommon today. In addition to the obvious advantage ofallowing for more transistors on a single chip, the smaller devicegeometries require less operating power and provide for faster switchingspeeds.

The preferred technology for state of the art submicron semiconductordevices is metal oxide silicon (MOS) transistors, which devices havehistorically allowed for ready scaling to smaller sizes as new submicronfabrication technologies are developed. MOS technology is approachingpractical scaling limits, however, and it is projected that conventionalMOS transistors cannot be scaled beyond 0.07 micrometers in theirminimum feature size. These practical limitations include well knownsemiconductor phenomena, such as hot electron injection, gate oxidetunneling, short channel effects, and sub-threshold leakage that arisewhen the features of the transistor are too close together to allowproper turn-on and turn-off behavior.

It is also essential in military and space applications of digitalelectronics to prevent ambient nuclear or solar radiation from affectingthe dynamic operation of switching devices. Switches based onsemiconductor materials are vulnerable to such radiation effects,however.

Therefore, a need exists for a submicron switching device that does notconsume excessive power, that has a fast switching response time, andthat can be scaled beyond the current practical limitations forsemiconductor switching transistors. The need also exists for asubmicron switching device that is largely unaffected by high doses ofparticle, electromagnetic or other radiation.

SUMMARY OF THE INVENTION

In one aspect, the present invention provides a nanomechanical switchcomprising a substrate and first, second and third electrodes formed onthe substrate. The third electrode includes a cantilever memberextending over the first and second electrodes. A voltage source iscoupled between the first and third electrodes, wherein the cantilevermember has an undeflected state when no bias is applied between thefirst and third electrodes, and a deflected state when a bias is appliedbetween the first and third electrodes.

In other aspects, the invention provides for logical circuits formed ofone or more such nanomechanical switches being connected together. Inanother aspect, the present invention provides for an integrated circuitcomprising a substrate, a power conductor, a ground conductor, an inputterminal and a logic circuit. The logic circuit comprises a plurality ofnanomechanical switches, at least one nanomechanical switch beingcoupled to said power conductor, and at least one nanomechanical switchbeing coupled to said ground conductor. Each such nanomechanicalswitches comprise a first electrode, a second electrode, and a thirdelectrode having a cantilever member extending substantially parallel tothe substrate and extending over the first and second electrodes. Thelogic circuit further comprises a voltage source coupled between thefirst and second electrodes, wherein the cantilever member has anundeflected state when no bias is applied between the first and thirdelectrodes, and a deflected state when a bias is applied between thefirst and second electrodes.

BRIEF DESCRIPTION OF THE DRAWINGS

The above features of the present invention will be more clearlyunderstood from consideration of the following descriptions inconnection with accompanying drawings in which:

FIGS. 1a and 1 b are a cross section of a first preferred embodimentswitch in the open state and the closed state, respectively;

FIGS. 1c and 1 d are plots of the response time of a first preferredembodiment nanomechanical switch;

FIG. 1e is a schematic representation of the first embodimentnanomechanical switch;

FIG. 1f is a schematic representation showing the first embodimentnanomechanical switch in a circuit;

FIGS. 2a through 2 j illustrate a preferred embodiment process formanufacturing a nanomechanical switch 10;

FIGS. 3a through 3 c illustrate a second preferred embodimentnanomechanical switch;

FIGS. 4a through 4 c illustrate a third preferred embodimentnanomechanical switch;

FIGS. 5a and 5 b illustrate multiple nanomechanical switches comprisinga common drain circuit;

FIG. 5c is a schematic representation of the multiple nanomechanicalswitches of FIGS. 5a and 5 b;

FIG. 5d is a schematic representation showing the multiplenanomechanical switches of FIGS. 5c in a circuit;

FIGS. 6a through 6 h illustrate a preferred embodiment complementarypair nanomechanical switch circuit;

FIGS. 7a and 7 b illustrate a second preferred embodiment complementarypair nanomechanical switch circuit;

FIGS. 8a and 8 b illustrate a preferred embodiment complementaryinverter;

FIGS. 9a through 9 c illustrate a preferred embodiment circuit;

FIG. 10 is a graph plotting tunneling current as a function of tunnelingdistance for preferred embodiment nanomechanical devices;

FIGS. 11a and 11 b illustrate a fourth preferred embodimentnanomechanical switch;

FIGS. 12a and 12 b illustrate a fifth preferred embodimentnanomechanical switch; and

FIG. 13 illustrates a preferred embodiment integrated circuit comprisedat least in part of nanomechanical switches.

DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS

The making and use of the various embodiments are discussed below indetail. However, it should be appreciated that the present inventionprovides many applicable inventive concepts that can be embodied in awide variety of specific contexts. The specific embodiments discussedare merely illustrative of specific ways to make and use the invention,and do not limit the scope of the invention.

The present invention provides for a nanomechanical switch 10 as shownin FIGS. 1a and 1 b. The switch is formed on substrate 2, preferablyformed of a bulk semiconductor, but alternatively formed of any suitablenon-conductive substrate such as glass, sapphire, ceramic, plastic, orthe like. With a ceramic substrate, it would be particularlyadvantageous to use a glazed ceramic in order to obtain a smooth enoughsurface for further processing of the device features. Quartz would alsomake a good substrate because of its relatively low dielectric constantcompared to material such as semiconductor, in order to preventformation of unwanted capacitance with the substrate. In otherembodiments, the switch may be formed on an insulating layer formed onthe surface of substrate 2. In the case of the semiconductor substrate,a silicon dioxide layer could be formed prior to further processing.

Formed atop substrate 2 are metallic pads 6 and 8, referred to herein asthe gate and drain, respectively. A source 4 is formed as an elongatedmetallic member 14 that is cantilevered over drain 8 and gate 6, andseparated from them by thickness t₁ and t₂, respectively. Note thatdrain pad 8 is thicker (i.e. taller) than gate pad 6. This is to preventcantilever 14 from contacting gate pad 6 when in its deflected state, aswill be explained in further detail below.

In the first preferred embodiment, the dimensions for gate 6 and drain 8are 0.1 microns by 0.1 microns. The dimension for drain 8 is preferably0.05 microns by 0.1 microns. The dimensions for oxide feature 3, whichacts as a hinge support, are preferably approximately the same as fordrain 8. As will be apparent to one skilled in the art when taking theteachings contained herein into account, variations to the preferredembodiments can be made depending upon the design parameters to beachieved.

Formed atop and supported by oxide feature 3 is source 4, includingcantilever 14 which extends substantially horizontally out from oxidefeature 3 over drain pad 8 and gate pad 6. Note that cantilever 14 isspaced apart from the top of drain pad 8 when in its normal orundeflected state, by a switch gap, denoted as t₁. In the preferredembodiments, t₁ is about 20 to 40 angstroms in the open or undeflectedstate. Preferably t₂ should be about 10 to 20 angstroms greater than t₁,or about 30 to 50 angstroms. Although shown as projecting outhorizontally, in alternative embodiments, cantilever may project outfrom source pad 4 at an angle to substrate 2 in order to, e.g. increaseor decrease the normal state gap between cantilever 14 and drain pad 8.Also contemplated within the scope of the invention, is a nanomechanicalswitch that is formed normal to the plane of the substrate, such as maybe formed on the sidewall of a deep trench in the substrate or a deeptrench formed in a layer of material deposited on the substrate.

Also shown is voltage source 12 connected between source pad 4 and gatepad 6. In FIG. 1a, voltage source 12 is at 0V. Source 4 and hencecantilever 14 are at zero bias with regard to gate pad 6. In FIG. 1b,however, voltage source 12 has been set to a positive value causing gatepad 6 to be positively biased with respect to source pad 4 andcantilever 14. This bias causes an electrostatic attraction between gatepad 6 and cantilever 14, thus pulling cantilever 14 downwards. As shown,because gate pad 6 is below the height of drain pad 8, cantilever 14contacts drain pad 8 as a result of the attractive pull. In this way, anelectrical circuit is completed between source pad 4 and drain pad 8.Note that the electrostatic attraction between gate pad 6 and cantilever14 persists even when current flows from source 4 through cantilever 14to drain 8. Note also that the electrostatic attraction is independentof the polarity of the voltage between source 4 and gate 6. FIG. 1eprovides a schematic representation of the switch 10, while FIG. 1fshows the switch 10 in a circuit.

A brief discussion about the performance characteristics of thepreferred embodiment switch is now provided. In the first preferredembodiment, cantilever 14 is constructed of aluminum and has a thicknessof approximately 25 nm, a width of approximately 100 nm and a length ofapproximately 200 nm. Such a cantilever would have a volume of 5E-16 ccand a mass of about 1.35E-18 Kg. Assume the desired switching frequencyis 2 GHz, then the switch response time must be 500 ps. This responsetime requires an approximate acceleration of the cantilever 14 at therate (2*t₁)/T₂ where t₁ is the switch gap (the distance betweencantilever 14 and drain pad 8, and T is the response time. For a switchgap of 4 nm, and a response time of 500 ps, the required accelerationwould be 3.2E10 m/s². Under such acceleration, the velocity ofcantilever 14 at the point it contacts drain pad 8 would beapproximately (2*t₁)/T or 16 m/s. Obviously, even faster switching timescould be obtained, depending on the attractive force between cantilever14 and gate pad 6, the mass of cantilever 14, the switch gap t₁, and theamount of voltage available to bias gate pad 6. For instance, the workthat must be extended to move preferred embodiment cantilever 14(mass*acceleration*distance) is 1.7E-16 Joules (1.35E-18 Kg*3.2E10m/s²*4 nm). This compares to the highest speed conventionalsemiconductor switches, which require switching energies in the range oftens of femtojoules. Assuming 2 watts of chip power were available,preferred embodiment cantilever 14 can be continuously switched at 2 GHzwith an operating power of 1.7E-16*2E9=0.34 microwatts. This would allow2/0.34E-6=6 million such switches to operate simultaneously with the 2watt power budget.

Note that because of the inherent resiliency of cantilever 14, theswitch will return to its open state (as shown in FIG. 1a) substantiallyas rapidly as the cantilever reaches its closed state. No biasing orvoltage is required to open the switch; cantilever 14 will return to itsundeflected state due to its spring constant once the gate voltage isremoved.

The approximate required gate voltage for voltage source 12 can readilybe determined from the formula F=CV²/t₂ where F is the required force tooperate the switch, V is the gate voltage, C is the gate capacitance,i.e. the capacitance between gate pad 6 and cantilever 14, and t₂ is thedistance between gate pad 6 and cantilever 14. In the above describedconfiguration, the gate capacitance C is about 1.8E-17 Farads (based onthe dimensions of the cantilever and an underlying gate of roughly 100nm by 100 nm), and the distance d is about 5 nm. A gate voltage of about3.5 volts would be sufficient to fully deflect cantilever 14, assuming acantilever vertical force (restoring force) constant of less than 16Newtons per meter. Even lower gate voltage can be used if slowerswitching speeds are acceptable or if a lighter or less stiff cantileveris employed.

Disregarding contact resistance, the on-state resistance of the switchis about one ohm, which compares very favorably with the on-stateresistance of about 92E3 ohms that would be expected for a 0.1micrometer wide transistor using conventional MOS technology. With sucha low resistance, the RC delay of the gate will be on the order of2.2E-17 F*20E3 ohms=0.4 psec.

FIGS. 1c and 1 d demonstrate the response time of switch 10 going froman open state (cantilever undeflected) to a closed state (cantileverdeflected) for a 3 volt gate voltage and a 2 volt gate voltage,respectively. Note that the cantilever deflects the full 40 angstroms(i.e. closes the switch gap) in roughly 400 picoseconds in FIG. 1c whena 3 volt gate voltage is applied. The response time increases to about900 picoseconds at the lower gate voltage of 2 volts, as shown in FIG.1d.

FIGS. 2a through 2 j illustrate a preferred embodiment process formanufacturing a nanomechanical switch 10. In FIG. 2a, two metal layers22 and 24 have been formed atop substrate 2. In the preferredembodiments, metal layer 22 is formed of platinum and metal layer 24 isformed of aluminum. Each layer is preferably 500 Angstroms thick and canbe thermally deposited. Alternative deposition techniques such assputtering, vapor deposition, and the like, as well as epitaxial growthof metal layers 22 and 24 are also contemplated.

As shown in FIG. 2b, a photoresist layer 26 is patterned atop metallayer 24 with patters 28 and 29. Pattern 28 is formed in order to createan etch between islands 30 and 32 after an etch step has been applied,as shown in FIG. 2c. Pattern 29 in photoresist layer 26 is included inorder to form island 30 symmetrically. In a separate process step, notshown, islands 30 and 32 are covered with a photoresist layer and theportion of metal layers 22 and 24 lying to the left of metallic island30 is exposed to an etchant and etched away. The resulting structure isthat illustrated in FIG. 2c. In an alternative embodiment, pattern 29could be omitted and islands 30 and 32 formed without the interveningsteps associated with pattern 29.

In a next process step, island 30 is covered with photoresist layer 34,as shown in FIG. 2d. and island 32 is exposed to an etchant thatselectively etches away metal layer 24, while leaving metal layer 22intact, as shown in FIG. 2e. In the preferred embodiment structure, HCIor KOH etchants may be employed to selectively etch away the aluminumlayer 24 without etching platinum layer 22. As illustrated in FIG. 2f,islands 30 and 32 are then covered with a deposited oxide layer 38, suchas silicon dioxide, and oxide layer 38 is then subjected to a chemicalmechanical polish step to form a smooth, planar surface 40, as shown inFIG. 2g. In other embodiments, a flowable oxide may be employed to formfirst oxide layer 38, which has the advantage of forming a relativelyplanar surface. Alternatively, plasma oxides could be employed. A secondoxide layer 42 is then formed atop the planar surface 40, as shown inFIG. 2h. Second oxide layer 42 can be formed of the same material asfirst oxide layer 38, although this is not necessary. In the preferredembodiments, second oxide layer 42 is selected for providing goodadhesion to metal layer 44 (FIG. 2i). For instance, if metal layer 44 isto be formed of aluminum, oxide layer 42 may be selected as aluminumoxide to provide good adhesion.

As will become apparent below, the thickness of second oxide layer 42defines the switch gap t₁. In other embodiments, second oxide layer 42could be eliminated if first oxide layer 38 could be grown or depositedwith enough control and precision to form first oxide layer 38 with thedesired thickness t₁ above island 30.

Metal layer 44 is then deposited atop second oxide layer 44 to formsource 4, including cantilever 14. As will be apparent to one skilled inthe art, source 4 and cantilever 14 can be formed through selectivedeposition of metal layer 44, or through subsequent mask and etchingsteps. Metal layer 44 is preferably aluminum or copper doped aluminum.The desired properties of metal layer 44 are low resistance, highresilience, and compatibility with metal layer 24 which forms the topsurface of drain 8 (shown as island 30 in FIGS. 2c through 2 k).Alternatively, metal layer 44 could be formed of platinum or some otherrefractory metal such as gold, nickel, paladium, tungsten or the like.Care should be taken in the selection of metal layer 24 and metal layer44 that the selected metals do not tend to form alloys with each otherand do not tend to stick together (from thermal bonding) as cantilever14 (metal layer 44) comes into contact with drain 8 (metal layer 24).Another advantage of refractory metals is that very thin films can beformed, while still being a continuous film. In the preferredembodiments, metal layer 24 and metal layer 44 are formed of the samerefractory metal, for the reasons discussed above. In other embodiments,other conductive materials, such as doped carbon or doped semiconductor,could be utilized in place of metal layers 22, 24, 44.

Also contemplated within the scope of the invention is laminated metallayers 22, 24, and 44, in which multiple sub-layers of different metals(such as platinum and aluminum) are sequentially deposited to form themetal layer. Laminated metal sub-layers tend to minimize warping anddeformity of the layer, thus allowing tight tolerances in the switch gapt₁ and also in t₂. The smaller the switch gap t₁, the lower theoperating voltage required to switch nanomechanical switch 10 from itsopen to closed state because of the higher field being generated acrossthe gap.

In a subsequent process step, oxide layers 38 and 42 are subjected to anetchant such as CF₄+O₂ or other well known plasma etchants. The desiredetchant will provide good lateral etching of the oxide layers in orderto etch oxide layers 38 and 42 back beneath metal layer 44, leaving agap between metal layer 44 and island 32, and between metal layer 44 andisland 30, as shown in FIG. 2j. The resulting structure provides forgate 6 formed from island 32, drain 8 formed from island 30, and source4 including cantilever 14 formed from metal layer 44. Although not shownfor clarity, appropriate interconnects will also be provided in order toconnect resulting switch 10 with other circuit components, includingvoltage source 12 between source 4 and drain 6.

FIGS. 3a and 3 b illustrate in cross section and FIG. 3c illustrates inperspective view, a second preferred embodiment nanomechanical switch10, in the open and closed states, respectively. In the second preferredembodiment, source 4 is formed of a metallic island upon which is formedcantilever 14, and gate 6 is formed between source 4 and drain 8. Aswill be apparent to one skilled in the art, the processing required forforming the second preferred embodiment structure is similar to thefirst preferred embodiment structure, although three metallic islandswould be formed, one each for the source 4, gate 6 and drain 8. Twoadvantageous features of the first preferred embodiment bear noting.First, by having the gate 6 spaced apart from the fulcrum for cantilever14, greater torque can. be obtained when gate 6 applies an electrostaticforce on cantilever 14. Another advantage is that the larger gap betweengate 6 and cantilever 14 may allow for more rapid and uniform etching ofoxide layers 38 and 42, then would be provided with the smaller gapbetween drain 8 and cantilever 14 of FIGS. 3a and 3 b.

Various alternative embodiment switches and circuits will now bedescribed with reference to FIGS. 4a through 9 c. For instance, FIG. 4aillustrates in perspective view an alternate layout for nanomechanicalswitch 10 in which the gate 6 and drain 8 are both positioned under andnear the free end of cantilever 14. FIG. 4b illustrates in elevationview from the side, and FIG. 4c illustrates in elevation view from thefree end of the cantilever, the same switch 10.

FIG. 5a provides a perspective view and FIG. 5b an elevation view of acommon drain, two switch circuit 10 and 10′, as illustratedschematically in FIG. 5c. The circuit has two sources 4 and 4′,including two cantilevers 14 and 14′, either of which will makeelectrical contact with common drain 8, under the control of gate 6 or6′, respectively. Only one source 4′ and one gate 6′ is illustrated inFIG. 5b, as the other source 4 and other gate 6 will be obscured whenseen from the side. FIG. 5c illustrates the common drain circuitschematically, while FIG. 5d shows the common drain circuit used in alarger circuit.

FIGS. 6a through 6 h illustrate a nanomechanical switch configured toprovide a CMOS type complementary response, as illustrated schematicallyin FIGS. 6d and 6 h. In FIG. 6a the switch 10 is shown in perspective.Control gate 6 is shown underlying cantilever 14 but in this embodimentcantilever 14 is electrically coupled to output pad 50, rather thanbeing formed from source pad 4. Instead source pad 4 is formed beneaththe free end of cantilever 14 and drain 8 is formed overlying the freeend of cantilever 14. FIGS. 6b and 6 c illustrate the switch 10 of FIG.6a in elevation view from the side and end-on view from the free end ofcantilever 14, respectively. As shown, in the undeflected state (i.e.gate 6 being unbiased with respect to cantilever 14), cantilever 14 isheld against drain 8 by the inherent spring tension of cantilever 14.Output pad 50 is thus connected to drain 8 and current will flow fromdrain 8 to output 50. As shown in FIGS. 6e, 6 f and 6 g, when a controlvoltage is applied to gate 6, an electro-static attraction between gate6 and cantilever 14 deflects cantilever 14 downward, breaking itselectrical contact with drain 8 and bringing cantilever 14 intoelectrical contact with source 4. In this case current will flow fromsource 4 to source 50 (or in the other direction depending on therespective voltage levels on source 4 and output 50). In this way,output 50 can be electrically connected to either the drain 8 or thesource 4. Setting drain voltage equal to a logical high and the sourcevoltage equal to a logical low will result in a CMOS type circuit as isknown in the art.

An alternative CMOS type circuit is illustrated in perspective in FIG.7a and in plan view (i.e. top down view) in FIG. 7b. As illustrated, twogates 6 and 6′ are positioned on either side of a laterally movingcantilever 14 which is connected to output 50. Note that unlike thepreviously discussed embodiments in which cantilever was deflectednormally to the plane of substrate 2, in the embodiment shown in FIGS.7a and 7 b, cantilever 14 moves in a plane substantially parallel to theplane of substrate 2. In an undeflected state, i.e. when neither gate 6nor gate 6′ is biased with respect to cantilever 14, the cantilever willbe positioned between gate 8 and source 4, but in electrical contactwith neither. When a control voltage is applied to the first gate 6,cantilever will be deflected toward it and will come into electricalcontact with drain 8, thus allowing current to flow between drain 8 andoutput 50. On the other hand, when a control voltage is applied to thesecond gate 6′, cantilever 14 will be deflected toward source 4, thuselectrically connecting source 4 and output 50 and disconnecting drain 8and output 50. Note that, as best shown in FIG. 7b, the gates 6 and 6′are spaced further apart from cantilever 14 than are source 4 and drain8 in order to ensure that cantilever 14 comes into electrical contactwith source 4 and drain 8 and does not short out against or come intoelectrical contact with gates 6 and 6′. An advantage provided by thisembodiment is that the output 50 can be put in a high impedance state,i.e. when neither gate 6 nor gate 6′ is biased with respect tocantilever 14, or when both gates 6 and 6′ are essentially equallybiased with respect to cantilever 14.

Yet another device is illustrated in FIGS. 8a (plan view) and 8 b(schematically), where two cantilevers 14 and 14′ are employed to form acomplementary inverter. A single gate control voltage is tied to twogates 6 and 6′ associated with cantilevers 14 and 14′ respectively. Theillustrated circuit will provide an inverted signal in response to aninput signal G1 being applied to gates 6 and 6′. As shown, eachcantilever has a fixed voltage source placed opposite its respectivegate. Fixed voltage source 52 is adjacent cantilever 14 opposite fromgate 6 and is tied to a 0V. Fixed voltage source 52′ is adjacentcantilever 14′ opposite from gate 6′ and is tied to 1V. Note thatvoltage sources 52 and 52′ are spaced further from cantilevers 14 and14′, respectively, than are gates 6 and 6′, respectively. Assume a zerovolt (or logical low) gate voltage GI is input to the circuit. Gate 6will electrostatically attract cantilever 14, which is held at a onevolt via source 4 and will deflect cantilever 14 to contact output pad50. Cantilever 14′, being held to a zero volt level will not beattracted to gate 6′ when it is at a logical low. On the other hand,when gate voltage G1 is set to a logical high voltage, cantilever 14′will be attracted and deflected toward gate 61, thus connecting output50′ to drain 8 at zero volts. At the same time, cantilever 14, beingheld at a logical high level by source 4, will no longer be deflectedsufficiently to contact gate 6. FIG. 8b schematically illustrates thecircuit.

Yet another device is illustrated in FIGS. 9a through 9 c. In thisembodiment, a logical AND function is achieved through serially gangingtwo nanomechanical switches 10, as shown in perspective in FIG. 9a. Inthis embodiment, the drain of the first switch is connected to thesource of the second switch. The first source 4 is deflected to contactcommon source/drain 54 when first gate 6 is high (i.e. biased withrespect to cantilever 14 of first source 4). Common source/drain 54 iscontrolled by second gate 6′ and connects to drain 8 when gate 6′ ishigh. A logical input at drain 8 will be coupled to source 4 only whenboth gates 6 and 6′ are high. FIG. 9b schematically illustrates thecircuit of FIG. 9a, and FIG. 9c provides a truth table for the circuitshowing its logical AND function.

In many applications, the gate to source voltages that are used tooperate the cantilever will be of the same magnitude as those appliedbetween the source and drain. It is necessary to arrange the dimensionsof the gate and drain such that the electrostatic force of the gate tothe cantilever is sufficiently greater than the force between the drainand cantilever so as to prevent the cantilever from being deflectedsimply by the voltage applied between the source and drain pads.Likewise, the restoring force of the cantilever must be sufficientlygreater than the electrostatic force between the drain and thecantilever when the cantilever is deflected onto the drain so as toprevent the cantilever from remaining in the deflected state when thegate voltage is removed.

The electrostatic force between two roughly parallel metals isapproximately CV²/d where C is the capacitance between the metals, V isthe potential difference between the metals, and d is the separationbetween them. Since in many applications, V and d will be similar forthe gate and drain voltages and for the cantilever separations, one wayto ensure that the drain to cantilever force is substantially less thanthe gate to cantilever force is to ensure that the capacitance betweenthe gate and the cantilever is substantially greater than the drain tocantilever capacitance. This is preferably accomplished by making thegate pad (electrode) much larger in surface area than the drain pad. Itis also noted that locating the drain pad between the cantilever hingeand the gate pad also reduces the effect of the drain voltage relativeto the gate due to the greater leverage of the gate force on thecantilever relative to the drain force.

In the above described embodiments, it has been assumed that physicalcontact between the cantilever and the drain was required in order togenerate source to drain current. As will be described in the followingparagraphs, actual contact is not required because the device can takeadvantage of electron tunneling. Electron tunneling is a quantumphenomenon in which an electron will cross an insulating barrier, suchas a vacuum gap or air gap, provided the barrier is sufficiently thincompared to the quantum or DeBroglie wave length of the electron in thebarrier material.

Referring now to FIG. 10 in conjunction with FIG. 3a, a graph shows therelationship between the tunneling current between cantilever 14 anddrain 8 as a function of the distance between the cantilever and thedrain. The tunneling current is a function of the surface area betweenthe electrodes, the work function of the electrodes, the voltage betweenthe cantilever and drain, and the distance between them. Assume thedimensions of drain pad 8 are 500 angstroms by 1000 angstroms the switchgap t₁ varies from 20 angstroms in the undeflected state to 5 angstromsin the deflected state, and that the cantilever to drain voltage is 2volts. For the preferred embodiment switch in which the gap material isvacuum, the work function for aluminum electrodes is 4 volts. Therelationship between tunneling current and switch gap t₁ is shown inFIG. 10. In the undeflected state, (i.e. t₁ at 20 angstroms) thetunneling current between cantilever 14 and drain 8 would be on theorder of 10E-18 amps—effectively an open circuit. In fact, this amountof off state tunneling current is much less than the off state leakagecurrent associated with current MOS transistors by many orders ofmagnitude. Note also that an undesired tunneling current may also occurbetween cantilever 14 and gate 6. The distance between cantilever 14 andgate 6 is preferably 40 angstroms, however. By extrapolating the plot ofFIG. 10, it will be clear that the tunneling current crossing the 40angstrom gap will be essentially non-existent.

By contrast, when the cantilever is brought close to drain 8 (say 5angstroms), in response to control gate 6, the tunneling currentincreases exponentially to approximately 1 microamp—clearly sufficienton-state current for typical logic circuits, even though cantilever 14and drain 8 are not in contact.

FIGS. 11a and 11 b illustrate a preferred embodiment switch which takesadvantage of the tunneling effect to allow for a lubricating layerbetween cantilever 14 and drain 8 in order to minimize the risks ofmechanical failure, alloy formation, thermal bonding, and the likearising from metal to metal contact. FIGS. 11a and 11 b are essentiallythe same as FIGS. 3a and 3 b, but with the addition of lubricating layer58 formed on the top surface of drain 8. Lubricating layer 58 ispreferably an oxide or polymer insulating layer that of approximately 5angstroms thickness. This layer provides a buffer between cantilever 14and drain 8 when the cantilever is in its deflected state. Even thoughlayer 58 is an insulating layer, as discussed above, the switch, whencantilever 14 is deflected, will be in an on state because of thetunneling current flowing across the 5 angstrom layer. In fact, becausethe work function for an oxide or polymer is less than that for vacuum,more tunneling current will flow across layer 58 than would flow acrossa vacuum gap of similar thickness. Thus many different insulators orconductors could be selected for the lubricating layer 58 withoutreducing the performance relative to a vacuum insulator. Any of theabove described embodiments will also exhibit the desired tunnelingphenomenon with appropriately chosen materials and dimensions.

The above embodiments have been essentially “MOS analogs” in which thecontrolling signal is a gate voltage. A “bipolar analog” embodimentcontrolled by current will now be described with reference to FIGS. 12aand 12 b. As shown in FIG. 12a, switch 60 comprises an emitter 64electrode including a cantilever 74, and a base electrode 66 andcollector electrode 68. Current flowing through base electrode 66creates a magnetic field surrounding the electrode, as shown in FIG.12b. This magnetic field induces a magnetic field in emitter electrode64, and in particular cantilever 74 and also in collector electrode 68.The induced magnetic fields in cantilever 74 and collector 68 areoriented with the polarity of the field created by current in baseelectrode 66. As illustrated, the induced magnetic north pole incantilever 74 will be complementary to the induced magnetic south polein collector electrode 68, thus resulting in a magnetic attraction thatwill cause cantilever 74 to deflect toward collector 68. It should benoted that the magnetic field emanating from base electrode 66 decreasesroughly with the distance r from the electrode (1/r). The distancebetween base electrode 66 and cantilever 14 must be fairly smalltherefore, in order to sufficient a magnetic field to be induced incantilever 14 and collector 68 to overcome the natural resiliency ofcantilever 14.

In the preferred embodiment magnetic switches, both cantilever 74 andcollector 68 should be formed of material with magnetic permeabilitymuch greater than one, such as ferro-magnetic or para-magneticmaterials, so as to have a strong magnetic field induced within them.Examples would include iron, nickel, cobalt, paladium, conductive alloysof these materials, and the like. Note that the use of multi-layermaterials may be particularly advantageous for obtaining both desirableconductivity and magnetic permeability in the cantilever, and thecollector and emitter. Preferably the multi-layer material would providefor a high conductivity surface layer, such as gold, covering a bulkmaterial with a high magnetic permeability, such as nickel.

An advantageous feature of the magnetic embodiments is that the magneticattraction is a function of the current in base electrode 66, as opposedto being a function of voltage. Therefore, magnetically activatedtransistors can be built with very low operating voltages. Baseelectrode 66 is preferably made of gold or other material with very lowresistivity in order to minimize the voltage required to generate basecurrent. Alternatively, by constructing base electrode 66 out of asuperconducting material, no energy would be consumed by the device inthe steady state, because no voltage would be required to maintain basecurrent.

Many variations to the described embodiments will be apparent to oneskilled in the art with the benefit of the teachings contained herein.For instance, the various switches and circuits described herein can becombined to form logical circuits and devices. The switches and circuitsdescribed herein can be fabricated using known semiconductor processingtechniques, and can hence be formed on a common substrate with classicalCMOS or NMOS switches and circuits. Whereas the embodiments have beendescribed with respect to a vacuum gap, Argon or some other noble gascould also be employed. An air ambient could also be employed if care istaken minimize effects such as corrosion.

FIG. 13 illustrates an integrated circuit 100 embodying aspects of theinvention. The integrated circuit includes a substrate 102 upon which isformed various circuit components. Signals can be supplied to andreceived from integrated circuit 100 by way of input/output ports 106.Power is supplied to integrated circuit 100 by way of power port 108 andground port 110, to which are coupled power conductor 112 and groundconductor 114, respectively for supplying power to circuit components.Included in the circuit components is nanomechanical logic circuit 116,Nanomechanical logic circuit 116 is formed of a series of interconnectednanomechanical switches as described above. Also shown in FIG. 13 ismemory array 118, preferably also formed of nanomechanical switchesconfigured as memory cells. Preferably, all circuits formed onintegrated circuit 100 are fabricated using nanomechanical transistorsin order to provide the benefits of speed, size, power savings, andradiation survivability discussed above. In some embodiments, however,semiconductor logic 120 could also be formed on substrate 102 using wellknown MOS, CMOS or bipolar semiconductor processes.

Various other modifications and combinations of the illustrativeembodiments, as well as other embodiments of the invention, will beapparent to persons skilled in the art upon reference to thedescription. It is therefore intended that the appended claims encompassany such modifications or embodiments.

What is claimed is:
 1. A nanomechanical switch comprising: a substrate;a first electrode formed on the substrate; a second electrode formed onthe substrate; a third electrode having a cantilever member extendingover the first and second electrodes; a voltage source coupled betweenthe first and third electrodes, wherein the cantilever member has anundeflected state when no bias is applied between the first and thirdelectrodes, and a deflected state when a bias is applied between thefirst and third electrodes; and a lubricating layer formed atop of thesecond electrode, the lubricating layer being adapted to conduct by wayof tunneling.
 2. A nanomechanical switch comprising: a substrate; afirst electrode formed on the substrate; a second electrode formed onthe substrate; a third electrode having a cantilever member extendingover the first and second electrodes; and a voltage source coupledbetween the first and third electrodes, wherein the cantilever memberhas an undeflected state when no bias is applied between the first andthird electrodes, and a deflected state when a bias is applied betweenthe first and third electrodes; wherein the cantilever is above spacedapart from the second electrode by between 10 and 40 angstroms in theundeflected state.
 3. A nanomechanical switch comprising: a substrate; afirst electrode formed on the substrate; a second electrode formed onthe substrate; a third electrode having a cantilever member extendingover the first and second electrodes; a voltage source coupled betweenthe first and third electrodes, wherein the cantilever member has anundeflected state when no bias is applied between the first and thirdelectrodes, and a deflected state when a bias is applied between thefirst and third electrodes; and an oxide feature formed on thesubstrate, on top of which is formed the third electrode.
 4. Ananomechanical switch comprising: a substrate; a first electrode formedon the substrate; a second electrode formed on the substrate; a thirdelectrode having a cantilever member, 2000 angstroms in length,extending over the first and second electrodes; and a voltage sourcecoupled between the first and third electrodes, wherein the cantilevermember has an undeflected state when no bias is applied between thefirst and third electrodes, and a deflected state when a bias is appliedbetween the first and third electrodes.
 5. The nanomechanical switch ofclaim 1 in which the lubricating layer is formed of an insulativematerial.